Re: [PATCH v3 1/2] power: supply: add input voltage limit property

[Pavel Machek <pavel@ucw.cz>]

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Hi!

> > >> For folks hacking on Pixel Cs (which is now outside of Google's official
> > >> support window for Android) and customizing their own kernel and userspace
> > >> this would be acceptable, but we wanted to expose this feature in the
> > >> power supply properties because the feature does exist in the Emedded
> > >> Controller firmware of the Pixel C and all of Google's Chromebooks with
> > >> USB-C made since 2015 in case someone running an up to date kernel wanted
> > >> to limit the charging power for thermal or other reasons.
> > >>
> > >> This patch exposes a new property, similar to input current limit, to
> > >> re-configure the maximum voltage from the external supply at runtime
> > >> based on system-level knowledge or user input.
> > >
> > > Could we get power input limit, instead?
> > >
> >
> > I'm open but I have some concerns, so lets discuss a bit about it :)
> >
> > According to the USB PD 2.0 specs if we limit the source at 15W we can get 5V/3A
> > or 9V/1.67A, if I am not mistaken the higher voltage caused problem since the
> > conversion to lower internal voltages generated more heat, so in this case
> > 9V/1.67A is not a valid value for us (maybe someone from ChromeOS can confirm
> > this?).

> Around Xmas are bad dates to start a discussion. I don't want this
> patch will be forgotten, so a gentle ping on your thoughts on this :)
> (just in case)

If someone can indeed confirm it is _voltage_ that is problematic,
then approach is ok with me. [But I'd not expect buck convertor
efficiency to differ greatly based on input voltage.]

Best regards,
									Pavel
									

> > >> +What:               /sys/class/power_supply/<supply_name>/input_voltage_limit
> > >> +Date:               Nov 2018
> > >> +Contact:    linux-pm@vger.kernel.org
> > >> +Description:
> > >> +            This entry configures the incoming VBUS voltage limit currently
> > >> +            set in the supply. Normally this is configured based on
> > >> +            system-level knowledge or user input (e.g. This is part of the
> > >> +            Pixel C's thermal management strategy to effectively limit the
> > >> +            input power to 5V when the screen is on to meet Google's skin
> > >> +            temperature targets). Note that this feature should not be
> > >> +            used for safety critical things.


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